It is generally known that a bicycle frame normally comprises a front triangle and a rear triangle.
The front triangle comprises, in turn, the following tubular elements:                a seat tube;        a top tube;        a down tube; and        a steering tube;        
whereas the rear triangle comprises, in turn, the following tubular elements:                two seat stays, which are integral to the seat tube; and        two chain stays, which are integral to the seat tube and to the down tube in correspondence to a bottom bracket axle.        
It is also generally known that bicycle frames can be manufactured using tubular elements made of different materials, such as steel, aluminium and titanium. Said tubular elements, then, can be assembled adopting assembling methods that are suitable for the materials chosen.
For example, steel is easy to be processed and repaired at low costs, but it is very heavy. Furthermore, steel is easily oxidable and, therefore, high-quality tubes must be subject to suitable treatments and be carefully painted.
In case steel tubes are used, they are assembled by means of braze welding operations, through MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.
For some years now, the use of bicycle frames made of aluminium has spread. Generally, an aluminium frame is less elastic, but stiffer and lighter than a steel frame. The smaller mechanical resistance of this material forces bicycle manufacturers to use tubes with a larger section and a greater thickness (hence the higher structural stiffness). The type of welding used for this material is TIG welding, or the elements are glued to one another.
Only a few dedicated manufacturers produce tubes made of titanium. These tubes are valuable, very light and have an intermediate elasticity coming between steel and aluminium. Titanium is very resistant and hardly oxidizes, so that it does not require any kind of paint. Though, this material is very expensive and hard to be processed, cut or welded.
Recently, different composite materials, which are usually developed for military needs, started to be used to manufacture bicycle frames. These materials are carbon, magnesium and thermoplastics. Though, these materials are very expensive and require sophisticated technologies and equipments, as well as a great expertise of the manufacturer. They allow manufacturers to optimize the dimensioning of each point of the frame, so as to obtain light structures with a suitable degree of stiffness. These materials are often used for monocoque bicycle frames, namely frames that are built without welding points as one single piece by means of a die.
The methods used to manufactures bicycle frames with these new materials, though, feature high costs and the use of sophisticated equipments.